Regulatory Interaction between the Cystic Fibrosis Transmembrane Conductance Regulator and HCO₃^- Salvage Mechanisms in Model Systems and the Mouse Pancreatic Duct

The pancreatic duct expresses cystic fibrosis transmembrane conductance regulator (CFTR) and HCO₃^- secretory and salvage mechanisms in the luminal membrane. Although CFTR plays a prominent role in HCO₃^- secretion, the role of CFTR in HCO₃ ^- salvage is not known. In the present work, we used molecular, biochemical, and functional approaches to study the regulatory interaction between CFTR and the HCO₃^- salvage mechanism Na ⁺/H⁺ exchanger isoform 3 (NHE3) in heterologous expression systems and in the native pancreatic duct. We found that CFTR regulates NHE3 activity by both acute and chronic mechanisms. In the pancreatic duct, CFTR increases expression of NHE3 in the luminal membrane. Thus, luminal expression of NHE3 was reduced by 53% in ducts of homozygote ΔF508 mice. Accordingly, luminal Na⁺-dependent and HOE694-sensitive recovery from an acid load was reduced by 60% in ducts of ΔF508 mice. CFTR and NHE3 were coimmunoprecipitated from PS120 cells expressing both proteins and the pancreatic duct of wild type mice but not from PS120 cells lacking CFTR or the pancreas of ΔF508 mice. The interaction between CFTR and NHE3 required the COOH-terminal PDZ binding motif of CFTR, and mutant CFTR proteins lacking the C terminus were not co-immunoprecipitated with NHE3. Furthermore, when expressed in PS120 cells, wild type CFTR, but not CFTR mutants lacking the C-terminal PDZ binding motif, augmented cAMP-dependent inhibition of NHE3 activity by 31%. These findings reveal that CFTR controls overall HCO ₃^- homeostasis by regulating both pancreatic ductal HCO₃^- secretory and salvage mechanisms.